1. Field of the Invention
The present invention generally relates to a tuning control apparatus of a receiver having an electronic tuner. More specifically, the present invention relates to a tuning control apparatus having an automatic search function.
2. Description of the Prior Art
A tuning control system of a receiver having an electronic tuner employing a voltage controlled variable reactance device has already been proposed and two types of such system set forth in the following are typically known. One of them is a frequency synthesizer system employing a phase-locked loop, while the other is a voltage synthesizer system employing a digital/analog converter. An example of such frequency synthesizer system is dislcosed in U.S. Pat. No. 4,081,752 issued Mar. 28, 1978 to Yasuaki Sumi. On the other hand, an example of such voltage synthesizer system is disclosed in U.S. Pat. No. 3,968,440 issued July 6, 1976 to George John Ahni, III.
A frequency sysnthesizer system employing a phase-locked loop controls the local oscillation frequency through a comparison of a frequency divided signal of the local oscillation frequency of the tuner with a reference signal obtained from a crystal controlled oscillator, thereby to control or determine the receiving frequency independently of the detection characteristic of the receiver. Therefore, the frequency synthesizer system involves a problem that even if the local oscillation frequency is controlled to be apparently tuned to the frequency of the electric wave the same does not necessarily become an optimum tuning point due to an error or a frequency drift and the like of a ceramic filter employed in an intermediate frequency stage subsequent to the tuner. Furthermore, another problem as set forth in the following is further involved. More specifically, although such frequency synthesizer system can be employed to perform an automatic search function, the same can be applied only to a receiver of a frequency band in the case of a tuning control apparatus which is capable of an automatic search function by the use of a frequency synthesizer system, when compatibility of shortening of a search time period and a stop accuracy is taken into consideration. Therefore, assuming different territories such as the Europe and the United States where channel space is different, separate tuning control appratuses need be designed and employed.
On the other hand, since a voltage synthesizer system is structured such that the local oscillation frequency is controlled based on a tuning state and a detection characteristic of a receiver, the same can be applied with ease to a frequency band of any channel space while an optimum tuning point can be readily attained. However, since a voltage synthesizer system fails to control the local oscillation freqeuency based on the reference signal of the crystal controlled oscillator, a problem is still involved in stability of reception as compared with a frequency synthesizer system employing a phase-locked loop.